1. Field of the Invention
This invention relates to cylinder blocks for use in engines and a method of making the same. More particularly, it relates to crankshaft bearings used in cylinder blocks, and a method of making the same.
2. Description of Related Art
Conventionally, as illustrated in FIG. 13, a cylinder block 101 includes a main body 103 constituting the upper part of the cylinder block, and bearing caps 105 disposed at the bottom of main body 103. These main body 103 and bearing caps 105 are both made of an aluminum alloy. These main body 103 and bearing caps 105 form a bearing section 107, in which a crankshaft (not shown) is rotatably supported. For reasons of rigidity, rotational balance and the like, this crankshaft is formed of an iron-based alloy (i.e., steel or cast iron). While the vehicle is running, the temperature of cylinder block 101 rises to about 100xc2x0 C. or above.
Since the aluminum alloy has a coefficient of linear expansion of about 22xc3x9710xe2x88x926/K and the iron-based member has a coefficient of linear expansion of about 12xc3x9710xe2x88x926/K, the bearing bore diameter of the bearing section expands to a greater extent than the outer diameter of the crankshaft when the vehicle is running to make the engine hot. Consequently, the clearance between the crankshaft and the bearing bore 109 is increased during running of the vehicle, so that the rotation of the crankshaft produces considerable vibrations and noises.
In order to prevent the production of such vibrations and noises, there has been proposed a method in which the bearing section 107 and the crankshaft are made of materials having almost equal coefficients of thermal expansion. For example, Japanese Patent Publication (JP-B) No. 6-86882 discloses a method in which the bearing section is wrapped with a cast iron-based material.
However, as contrasted with the specific gravity (2.7 g/cm3) of aluminum, the specific gravity of steel is 7.8 g/cm3 and the specific gravity of cast iron is 6.9 g/cm3. Thus, since the specific gravities of iron-based materials are about 2.5 to 3 times higher than that of an aluminum alloy, the wrapping of the bearing section with a cast iron-based material has a problem in that the weight of the cylinder block itself is unduly increased.
Moreover, the wrapping material is an iron-based material which is dissimilar to the aluminum alloy constituting the parent material, its adhesion to the aluminum alloy member is not always satisfactory.
Furthermore, the bearing section must be subjected to a final finishing step for forming.a bearing bore by machining. However, the iron-based material used as the wrapping material is so hard that its machinability is not satisfactorily good.
It is an object of the present invention to solve the above-described problems by providing a cylinder block which can reduce the vibrations and noises produced from the engine at high temperatures and has a light weight, as well as a method of making such a cylinder block.
According to the present invention, there is provided a cylinder block comprising a main body of the cylinder block, a bearing attached to the underside of the main body, and a crankshaft supported rotatably in a bearing section formed by the underside of the main body and the bearing, wherein an aluminum alloy layer is formed in the sliding portion of the bearing section, the region adjacent to the aluminum alloy layer consists of a composite material, and the coefficient of thermal expansion of the composite material is lower than that of the aluminum alloy layer formed in the sliding portion.
The bearing section of the aforesaid cylinder block is provided with a bearing bore for a crankshaft, and the inner surface of this bearing bore serves as a sliding surface on which a crankshaft rotates. The sliding portion including the sliding surface consists of an aluminum alloy layer formed, for example, of ADC12 (aluminum diecasting material), and the region adjacent to the sliding portion consists of a composite material.
When the engine becomes hot, for example, during running of the vehicle, the sliding portion tries to expand outwards by the action of heat. However, since the adjacent region has a lower coefficient of thermal expansion than the sliding portion, the sliding portion is forced back inwards by the composite material constituting the adjacent region, so that the expansion of the bearing bore is eventually controlled. Consequently, the clearance between the crankshaft and the bearing bore is not appreciably increased, thus permitting the vibrations and noises produced from the engine during running of the vehicle to be kept on a low level. Moreover, since the bearing section is composed of an aluminum alloy layer and a composite material using an aluminum alloy, the weight of the cylinder block can be reduced and this, in turn, contributes to a reduction in the weight of the whole vehicle. Furthermore, since the composite material is composed partly of an aluminum alloy, it can be simply prepared at low cost. On the other hand, the sliding portion consists of an aluminum alloy layer and hence exhibits good machinability during final finishing by machining of the inner surface of the bearing bore.
As the aforesaid composite material, there may preferably be used a composite material formed by impregnating a compression-molded preform with a molten aluminum alloy. The aforesaid preform is preferably prepared by compression molding of a particulate material, a fibrous material, or a mixture of a particulate material and a fibrous material. As the aforesaid particulate or fibrous material, there may preferably be used any of various ceramic materials such as oxides, carbides and nitrides.
Useful oxides include, for example, silica (SiO2), alumina (Al2O3), mullite (Al2O3xe2x80x94SiO2), spinel (MgOxe2x80x94Al2O3), magnesia (MgO) and calcia (CaO). Preferred examples of carbides include silicon carbide (SiC), and preferred examples of nitrides include silicon nitride (Si3N4), aluminum nitride (AlN) and boron nitride (BN).
Owing to the use of particular and/or fibrous materials, the aforesaid composite material brings about an improvement in the rigidity and toughness of the bearing section. Moreover, an aluminum composite material having high rigidity can be obtained by using a particular or fibrous material having a high modulus of longitudinal elasticity (Young""s modulus), for example, a modulus of longitudinal elasticity higher than the modulus of longitudinal elasticity of steel (i.e., 2.1xc3x971011 N/m2)
The method of making a cylinder block in accordance with the present invention comprises the step of preparing a preform by adding a silica type additive to at least one of a particulate material and a fibrous material, and compression-molding the resulting blend; and the step of casting a molten aluminum alloy into said preform and thereby fabricating a bearing section in which an aluminum alloy layer is formed in its sliding portion for a crankshaft and the region adjacent to the sliding portion consists of a composite material.
Preferably, the particulate material has an average particle diameter of 10 to 500 xcexcm, the fibrous material has an average fiber diameter of 1 to 10 xcexcm, and the preform has a volume fraction of 15 to 40%. The particulate or fibrous material may comprise at least one material selected from oxides such as silica, alumina, mullite, spinel, magnesia and calcia; carbides such as silicon carbide; and nitrides such as silicon nitride, aluminum nitride and boron nitride. The aforesaid volume fraction is defined as the proportion of the volume of the particulate and fibrous materials to the total volume of the compression-molded preform. This can be determined by calculating the bulk specific gravity of the preform from its volume and weight, dividing this specific gravity by the true specific gravity-of the particulate and fibrous materials added, and expressing the quotient as a percentage.
According to the above-described method, the surfaces of the particular material and/or the fibrous material are modified by the addition of the silica type additive, so that the pressure at which the molten aluminum alloy begins to penetrate into the preform is 1 MPa or less. In the fabrication of the composite material, therefore, the preform can surely be impregnated with the molten aluminum alloy in a short time. Accordingly, the length of time required to make a cylinder block can be reduced. Moreover, the composite material constituting the bearing section can be fully impregnated with the aluminum alloy.
Since the cylinder block of the present invention is made by using an aluminum alloy, it has a light weight and hence contributes to a reduction in the weight of the whole engine. Moreover, even if the cylinder block becomes hot, the bearing bore diameter is not appreciably enlarged and, therefore, the clearance between the crankshaft and the bearing bore is not increased. Consequently, even during running of the vehicle, the vibrations and noises produced from the engine can be kept on a low level.